1. Field of the Invention
The present invention relates to improvements in lithium-ion batteries or non-aqueous electrolyte secondary batteries such as polymer batteries. More specifically, the invention relates to a battery structure that is capable of stabilizing a non-aqueous electrolyte secondary battery quickly even when the non-aqueous electrolyte secondary battery develops an abnormal condition for some reason, whereby high reliability can be achieved even with a high capacity, high power battery construction.
2. Description of Related Art
Mobile information terminal devices such as mobile telephones, notebook computers, and PDAs have become smaller and lighter at a rapid pace in recent years. This has led to a demand for higher capacity batteries as the drive power source for the mobile information terminal devices. With their high energy density and high capacity, non-aqueous electrolyte batteries that perform charge and discharge by transferring lithium ions between the positive and negative electrodes have been widely used as the driving power source for the mobile information terminal devices. However, the non-aqueous electrolyte batteries have not yet sufficiently satisfied the requirements.
Moreover, utilizing their characteristics, applications of non-aqueous electrolyte batteries, especially Li-ion batteries, have recently been broadened to middle-size and large-size batteries for power tools, electric automobiles, hybrid automobiles, etc., as well as mobile applications such as mobile telephones. As a consequence, demand for higher battery reliability has been on the rise, along with demand for increased capacity and higher output power.
In order to improve the reliability of lithium-ion batteries, a polyolefin-based porous film has been used as a separator interposed between the positive and negative electrodes. In this case, a separator shutdown function operates when the battery temperature abnormally rises, so the battery reliability can be improved.
Nevertheless, even in the case of using a polyolefin-based separator, if the battery temperature greatly rises above the melting point of the polyolefin, the separator shrinks so that the positive and negative electrode active material layers can come into contact with each other, causing short circuiting. As a consequence, a large current flows through the portion where the short circuiting has occurred, and therefore, the temperature rises locally, causing a thermal decomposition reaction between the positive electrode active material and the electrolyte solution. In view of these problems, the key factors for improving the reliability of lithium-ion batteries are such technologies as improving thermal stability of the positive electrode active material, making the electrolyte solution inflammable, and preventing separator shrinkage.
Polyolefin-based porous films that are widely used for the separator in a lithium-ion battery melt and shrink at a low temperature about 120° C. to 160° C. when the battery temperature rises. Taking this into consideration, improving separator reliability is believed to be most effective among the above-mentioned technologies to improve the reliability of the battery. In this case, it is necessary to prevent separator shrinkage even at high temperatures while maintaining the separator's shutdown function. One of such technologies is described in Japanese Unexamined Patent Publication No. 10-50287, which discloses a technique of stacking a heat-resistant microporous film over a surface of the separator in order to prevent shrinkage of the separator at high temperatures and thereby improve battery reliability.
However, when the battery temperature rises above the melting point of the polyolefin-based separator, the positive electrode active material of the non-aqueous electrolyte secondary battery reacts with the electrolyte solution in a charged state or the positive electrode active material itself undergoes thermal decomposition and generates oxygen, so the interior of the battery is brought into a very instable condition. As a consequence, even when the separator shrinkage at high temperatures is prevented by stacking a heat-resistant microporous film over a surface of the separator, battery reliability cannot be ensured sufficiently as long as the potential of the positive electrode is kept high. For this reason, there is a demand for technology to quickly lower the potential of the positive electrode while preventing separator shrinkage at high temperatures.